In a wireless communication system, a variety of errors are inherent to radio frequency (RF) signal propagation over the air interface, thus making data transmission difficult. Because a radio frequency (RF) channel experiences multi-path fading, path loss increases proportional to distance between a mobile station (MS) and a Base Station (BS), etc. as well as thermal noise modeled as Additive White Gaussian Noise (AWGN), it is more difficult to transmit a signal reliably.
To ensure transmission reliability against various changes in channel status and errors during wireless communication, techniques such as 1) Forward Error Correction (FEC) or channel coding, and 2) Automatic Repeat reQuest (ARQ) or Hybrid Automatic Repeat reQuest (HARQ) are widely used.
The HARQ scheme serving as one of error compensation methods for guaranteeing communication reliability is achieved by combining FEC and ARQ. In more detail, the HARQ scheme recognizes whether a physical (PHY) layer includes an error incapable of decoding received data. If the error occurs, the HARQ scheme requests retransmission of data, resulting in an increase in signal transmission/reception performance. The HARQ scheme attempts to perform error checking of the received data, and determines whether to retransmit data using an error detection code. If an error of the received data has been detected in the error detection process, a receiver transmits a negative acknowledgement (NACK) signal to a transmitter. In this case, the transmitter having received the NACK signal transmits appropriate retransmission data according to the HARQ mode.
The HARQ scheme is applied to unicast data transmission on uplink or downlink, and a detailed description thereof is shown in FIG. 1.
FIG. 1 is a flowchart illustrating an HARQ transmission procedure on downlink.
Referring to FIG. 1, a base station (BS) transmits data (or burst) to a mobile station (MS). The MS having received data from the BS transmits a NACK signal on the condition that an error occurs in the data or an information block cannot be decoded. If no error occurs in data and an information block can be decoded, the MS transmits an ACK signal to the BS in step S120. Here, the ACK signal means success of data transmission, and the NACK signal means failure of data transmission and also indicates a data retransmission request. If the BS has received the ACK signal from the MS, the BS need not retransmit data to the MS. However, if the BS receives the NACK signal from the MS or if the BS does not receive ACK/NACK signals for a predetermined period of time, the BS retransmits data to the MS in step S130.
In this case, the ACK/NACK signals are processed for an ACK/NACK time delay serving as a processing time on the basis of a data transmission time of the BS, and are then transmitted. In this case, the ACK/NACK signals are transmitted through an HARQ feedback channel. In this case, the HARQ feedback channel may include at least one ACK/NACK signal, may include at least one orthogonal frequency division multiplexing (OFDM) symbol in a time domain, and may include at least one subcarrier in a frequency domain. In addition, one HARQ feedback channel signal may be FDM (frequency division multiplexing)-, TDM (time division multiplexing)-, and CDM (code division multiplexing)-processed and then transmitted. The HARQ method for data transmission from the BS to the BS can be equally applied even to data transmission from the MS to the BS.
As described above, although methods for transmitting feedback information such as HARQ ACK/NACK in a single network have been proposed in the conventional art, a method for transmitting feedback information on the condition that a specific device (MS or BS) uses a heterogeneous network (or a plurality of wireless access technologies) has not been proposed yet.